The present invention relates to a method for heat-treatment of a strip in a continuous annealing installation.
Various methods for cooling a strip with a cooling roll in a continuous annealing installation have been heretofore proposed. By way of example, in Laid-Open Japanese Patent Specification No. 58-96824 there is disclosed a method for cooling a strip with a cooling roll whose roll diameter fulfills a certain relation. This prior invention relates to a cooling roll for a strip, and according to the invention the roll diameter was determined on the basis of an amount of temperature drop of a strip which is cooled by a single roll. More particularly, it is disclosed that in the case where an amount of cooling with a single roll is 20.degree. C. or less, it becomes difficult to apply the cooling roll to a practical machine because cooling efficiency is poor and hence the number of cooling rolls is increased. Also it is disclosed that in the case where an amount of cooling with a single roll is 150.degree. C. or more, uneven cooling is apt to occur in a strip, and so it is difficult to produce a good strip.
On the basis of such recognition, in Laid-Open Japanese Patent Specification No. 58-96824, a heat transmission model is set up, assuming that an amount of heat released from a strip Q.sub.s and an amount of heat transmission between a strip and a roll Q.sub.r represented by the following Formulae (1) and (2) have equal values, the value of .DELTA.T.sub.s is substituted in Formula (3), and the relation among a roll outer diameter D, a heat transfer amount K, a strip thickness t and a line speed L.sub.s is defined as represented by Formula (4). ##EQU1##
The inventors of this invention repeated experiments more than several hundred times with respect to the method for heating and/or cooling a strip with a roll, similarly to the inventor of the above-referred prior invention, and as a result it was seen that the condition disclosed in Laid-Open Japanese Patent Specification No. 58-96824 was not yet sufficient. For instance, in some cases temperature unevenness occurred in a strip after cooling, or in other cases during cooling, a strip was extremely deformed, resulting in yielding, and corrugation-shaped strain or the so-called cooling buckle was produced.
With regard to the causes of these phenomena, the inventors of this invention analyzed in detail several hundred experimental data for heating and/or cooling by means of a roll, and as a result, it was found that a contact state between a roll and a strip would largely effect the temperature unevenness after cooling (or heating) of the strip and the temperature unevenness is greatly governed by bending of the roll caused by the weight of the roll itself, the weight of thermal medium flowing through the roll and the strip tension.